This R03 proposal was written in response to PA-11-049, Women and Sex/Gender Differences in Drug and Alcohol Abuse/Dependence. Growing evidence shows that drug-dependent women express greater negative affect than men (1, 2, 3, 4) and negative emotional states such as stress and depression are more likely to trigger craving and relapse in women (5, 6). Research in males has shown that aversive and depressive-like states are mediated, in part, by the neuropeptide dynorphin, an endogenous ligand that acts at kappa opioid receptors (7). Chronic exposure to drugs of abuse promotes the synthesis and release of dynorphin that is coincident with the emergence of depressive-like effects (8, 9, 10). The purpose of this proposal is to examine sex differences in the regulation and role of kappa-opioid receptors in mediating negative affective states in rats. Understanding the biological basis of these differences is crucial to developing better treatments for both men and women. Chartoff and colleagues have begun to examine the role of kappa-opioid receptors in males and females using intracranial self-stimulation (ICSS), an operant conditioning paradigm that is sensitive to increases or decreases in reward function in real time. Kappa- opioid receptor agonists increase stimulation thresholds in ICSS, which is indicative of a decrease in reward function (anhedonia). Preliminary results show that females are less sensitive to the threshold-increasing effects of the kappa-opioid receptor agonist U50,488. Given that the majority of sex differences have been linked to actions of circulating gonadal steroid hormones, this proposal uses two complementary approaches to test the role of gonadal hormones in kappa-opioid receptor function. First, the effects of U50,488 on ICSS thresholds will be determined in gonadectomized rats. If gonadectomy abolishes the sex difference in the depressive-like effects of KOR activation, then it is likely that activational effects of gonadal steroid hormones are required. Although a fairly basic question, it is fundamental to the understanding of mood dysfunction in drug addiction. Second, the role of sex and gonadal steroid hormones on kappa-opioid receptor mRNA levels and U50,488-induced coupling to downstream effectors will be determined within brain regions that regulate reward function. If receptor levels or coupling are modulated by sex or sex hormones, then it is likely that differences in the behavioral effects of kappa-opioid receptor activation are due, in part, to receptor pharmacodynamics. Data from these studies will lead to a better understanding of how kappa-opioid receptors regulate affective states in males and females and will form a foundation for future research on sex differences in the role of kappa-opioid receptors in drug addiction.